The commercial production of various types of paper requires the use of recycled paper as a source of papermaking fibers due to the expense of virgin fibers. Prior to using such secondary fiber sources for making a commercial product, it is necessary to treat the fiber source to remove unwanted chemical constituents which adversely affect the quality of the final paper product. The most notable contaminants to be removed are inks or dyes which adversely affect the color and brightness of secondary fibers used as a feedstock. Ink deposits on paper are extremely thin and roughly have a thickness of only about 0.0001 inch. Chemically, the inks are generally a mixture of pigment or organic dye, binder, and solvent. Some inks also contain metallic driers, plasticizers, and waxes to impart desired properties. Hence their chemical make-up can be very complex. However, inks are not to be equated with other additives or contaminants such as varnishes, sizes, and plasticizers, which are chemically and physically of a different nature as those skilled in the art of deinking will appreciate.
The prior art has addressed secondary fiber clean-up generally by subjecting secondary fiber sources to a variety of treatments. The most common form of treatment is chemical wet deinking. For example, U.S. Pat. No. 3,098,784 to Gorman teaches a process for deinking printed paper wherein the printed paper is slurried in water containing 0.2-5.0 percent (based on the weight of the paper) of a water-soluble non-ionic surface active agent at a temperature of from about 90.degree.-180.degree. F. The treatment is carried out in standard pulp fiberizing equipment wherein the paper stock is reduced to substantially individual fibers. U.S. Pat. No. 3,179,555 to Krodel et al. teaches an aqueous method using certain salts to induce a change in zeta potential between the ink particles and the material particles. A detergent is used to emulsify the separated ink particles, enabling them to be removed from the pulp by washing. U.S. Pat. No. 3,377,234 to Illingworth teaches a deinking agent for use in an aqueous solution comprising an admixture of alkyl sulfates, alkyl aryl sulfonates, and sodium polyphosphate. U.S. Pat. No. 1,422,251 (1922) to Billingham teaches disintegration of secondary fiber sources as a preparation for wet deinking. U.S. Pat. No. 2,018,938 (1935) to Wells teaches a wet deinking method wherein the waste paper is disintegrated in a soapy solution with a rod mill. U.S. Pat. No. 2,916,412 (1959) to Altmann et al. teaches a wet deinking method whereby slurried waste paper (3.25% consistency) is coarsely pulped and thereafter refined at a temperature below 112.degree. F. to shatter the ink from the fibers. German Pat. No. 2,836,805 (1979) teaches slurrying waste paper in a pulper at a 3-5% consistency in the presence of electrolytes to cause the fibers to swell, thereby causing the inks to crumble. However, these and other wet deinking processes can be expensive and produce large quantities of sludge, which creates a disposal problem. In addition, there are certain types of papers which cannot be successfully deinked at all by conventional wet methods because they are chemically unreactive with the deinking agents.
Other treatments of secondary fibers have been directed toward separating other contaminants besides inks from the secondary fibers, such as plastic coatings and miscellaneous particulates. For example, French Pat. No. 1295608 (1961) teaches recovery of waste paper coated with synthetic materials or plastic films by wetting the waste paper and subjecting the slurry to attrition in a beating device. The hydrophobic plastic particles can be separated from the hydrophylic fibrous material, which has been disintegrated by the attrition mill into particles (fibers) which are smaller than the plastic particles. British Patent No. 940,250 (1963) teaches a method for recovering fibrous materials from waste paper products which have been coated with synthetic resins in the form of a rigid film. The waste material is exposed to vigorous mechanical treatment in the presence of less than 70 weight percent water to fiberize the material, while leaving the synthetic resin film in relatively large pieces. British Patent No. 1228276 (1971) teaches a method for recovering fibrous material from plastic coated or plastic-containing waste paper. The waste paper is fiberized in water whereby the plastic separates from the fibers in small particles. The plastic particles are then separated from the fibers. A Russian article entitled "Dry Comminution of Waste Paper", M. V. Vanchakov, V. N. Erokhin, M. N. Anurov (Jan. 14, 1981) teaches dry grinding of waste paper in a hammermill as a pretreatment prior to a hydropulper to separate large contaminants such as fasteners, cloth, polyethylene film, and others. The ground material was passed through separator screens having 4 mm. and 8 mm. diameter holes and the fractions passing through the screens were defiberized in a hydropulper. However, as suggested previously, none of these methods are directed to deinking. All are concerned with removal of plastic films and coatings, which separate out as relatively large pieces. Also, except for the Russian article, all of these methods use water and accordingly are not suggestive of a dry process. On the other hand, the Russian article does not suggest deinking, but rather is directed toward removal of large contaminants rather than fines.
Still other prior methods of treating waste papers use different approaches. For example, U.S. Pat. No. 3,736,221 (1973) to Evers et al. teaches a method for making shaped bodies from waste paper by fiberizing the waste paper in a hammermill, coating the fibers with an aqueous binder, compressed under pressure, and baked. No effort is made to remove the ink from the waste paper. U.S. Pat. No. 4,124,168 (1978) to Bialski et al. teaches a method for recovering different types of wastepaper from a mixed source by fragmenting the source materials and separating the various components by their fragmentability. This method only serves to classify various types of waste paper present in a mixed sample and does not attempt to remove the ink from the waste paper. German Patent No. 1097802 (1961) teaches a method for reclaiming waste paper by tearing the paper and cleaning it, crimping and rolling the torn paper in a practically dry state, and defibering in the dry state, optionally in the presence of dry steam. This method seeks to overcome difficulties in fiberizing waste papers coated with hydrophobic materials which do not respond well to aqueous methods. There is no teaching, however, that inks can be removed by such a dry treatment.
Accordingly there remains a need for a deinking process which avoids or minimizes sludge formation and chemical costs. Although various prior art treatments of waste paper have attempted to satisfy this need, none of the methods have been successful.